levels, which may remain quite cold. A warming of10°C per day at the 500-hPa level is not unusual. Sucha rate of warming is not entirely due to subsidence butprobably has a considerable contribution from warmadvection. However, continuity considerations suggestthat the convergence in the 400- to 200-hPa stratumproduces some sinking and adiabatic warming in thelower troposphere. See figure 3-9.Thus, in the building of anticyclones, there must bea piling up of air at high levels due to horizontal velocityconvergence in the 400- to 200-hPa stratum, whichresults in the stratospheric cooling observed withdeveloping anticyclones. Insufficient outflow at veryhigh levels results in an accumulation of mass. This isroughly the mechanism thought to be responsible for thedevelopment of high-pressure systems. The high-levelincrease of mass overcompensates the low troposphericdecrease of density, and the high-level effect thusdetermines the sign of increase of pressure at the surfacewhen highs are intensifying. See figure 3-9.The development of anticyclones appears to be justthe reverse of the deepening of cyclones. Outside ofcold source regions, and frequently in cold sourceregions, high-level anticyclogenesis appears to beassociated with an accumulation of mass in the lowerstratosphere accompanied by ceding. In many casesthis stratospheric cooling maybe advective, but morefrequently the cooling appears to be clearly dynamic;that is, due to the ascent of air resulting from horizontalconvergence in the upper troposphere.Studies of successive soundings accompanyinganticyclogenesis outside cold source regions showprogressive warming throughout the troposphere. Thisconstitutes a negative contribution to anticyclogenesis.In other words, outside of cold source regions, duringanticyclone development, the decrease in DENSITY inthe troposphere is overcompensated by an increase inFigure 3-9.-Vertical circulation over developing high.3-13
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